biotechnical measures for erosion and sediment control (pdf)

CONTENTS

Page List of Figures Introduction……..……….……………………………………………………………………………………………. 4.1 Principles of Biotechnical Slope Protection…………………………………………………………………………… 4.1 Planning Considerations………………………..…………………………………………………………………….. 4.2 Plant Materials…………………..…………………………………...………………………………………………. 4.3 Live Fascines……………………………………..…………………………………………………………………… 4.5 Brush Mattress………………………………………………………………………………………………………… 4.7 Live Stakes……………………………………………………………………………………………………………. 4.9 Brush Layer……………………………………………………………………………………………………………. 4.13 Live Cribwall………………………………………………………………………………………………………….. 4.15 Tree Revetment………………………………………………………………………………………………………… 4.17 Branchpacking…………………………………………………………………………………………………………. 4.19 Fiber Roll………………………………………………………………………………………………………………. 4.21 References

SECTION 4 BIOTECHNICAL MEASURES

FOR EROSION AND SEDIMENT CONTROL

Section prepared by: Donald W. Lake Jr., P.E., CPESC, CPSWQ Engineering Specialist New York State Soil & Water Conservation Committee And John A. Dickerson, Plant Materials Specialist USDA Natural Resources Conservation Service Syracuse, NY

List of Figures Figure Title Page

4.1 Vegetated Rock Gabion Details…………………………………………………………………… 4.4

4.2 Live Fascine Details.………………………………………………….…………………………… 4.6

4.3 Brush Mattress Details..………………..………………………………..………………………… 4.8

4.4 Live Stake Details……………………..…………………………………………………………… 4.10

4.4A Live Stake Construction Specification……………………………………………………………. 4.11

4.5 Brush Layer Details……………………………………………………………………………….. 4.14

4.6 Live Cribwall Details……………………………………………………………………………… 4.16

4.7 Tree Revetment Details...…………………………………………………………………………. 4.18

4.8 Branchpacking Details…………………………………………………………………………….. 4.20

4.9 Fiber Roll Details…………………………………………………………………………………. 4.22

August 2005 .1 New York Standards and Specifications For Erosion and Sediment Control

Introduction Biotechnical slope protection is the specialized use of woody plant materials to stabilize soil. As noted in Section 1, one of the factors that affects erosion is vegetative cover. The more cover soil has, the more protected it is from the attacking forces of rainfall and runoff. Also working to hold the soil in place is the root mass that vegetation produces. Biotechnical measures generally combine basic engineering principles with plant science to create a system of stability for critical areas such as streambanks or roadside slopes. These systems may combine structural measures, such as those detailed in Section 5, with woody plants and shrubs to effect a strengthening of the soil structure and improved vegetative cover to resist surface erosion. There are many advantages to biotechnical slope protection measures: • they are often less expensive to install • they do not require specialized skills to install • generally, heavy equipment is not required • they are environmentally compatible • they provide a natural aesthestic appearance • they provide wildlife habitat and cover • they can be self repairing during and after stress • they use natural/native materials On the other hand, there are some disadvantages to these measures:

• higher risk due to less control with vegetation compared to structural practices

• require higher maintenance attention • need an establishment period • more sensitive to seasonal changes Biotechnical slope protection is actually an old technology. These techniques have been practiced for centuries in Europe. The Natural Resource Conservation Service used and promoted this technology in the 1940’s in Vermont on the Winooski River and also in New York on Buffalo Creek, where plant materials (willows) were used in combination with rock riprap, concrete slabs, pinned rock, and cellular modules to halt streambank erosion. These biotechnical approaches are being “rediscovered” primarily due to their cost effectiveness over more traditional structural measures and for their environmental compatiblity, aesthetics, and wildlife benefits. There are many areas in towns and counties in New York that

experience erosion on streambanks or sloughs on roadside slopes that could be controlled with biotechnical protection measures. The low cost and ease of installation is very attractive to units of government and highway departments looking to maximize their budget dollars. Principles of Biotechnical Slope Protection Generally a biotechnical slope protection system consists of both a structural or mechanical element and vegetative elements working together to stabilize a site-specific condition. Structural components are employed to allow establishment of vegetative elements, while at the same time providing a level of protection for stability. The vegetative components are not just landscaping plantings for a structural project; they also perform a functional role in preventing erosion by protecting the surface, while also stabilizing soil by preventing shallow mass movements. Woody plant materials (usually dormant shrub willow branches) are placed into the soil in ways that provide an immediate degree of stability to the slope. As the branches take root and grow, the slope becomes more and more resistant to failure by shallow mass movements due to:

1. Mechanical reinforcement from the root system, 2. Soil moisture depletion through transpiration and interception, and 3. Buttressing and soil arching action from embedded stems.

The vegetation also tends to prevent surficial (surface or rainfall) erosion by:

1. Binding and restraining soil particles in place, 2. Filtering soil particles from runoff, 3. Intercepting raindrops, 4. Retarding velocity of runoff, and 5. Maintaining infiltration.

As the stability improves, native vegetation will volunteer, helping to blend the site into the surroundings.

BIOTECHNICAL SLOPE PROTECTION MEASURES FOR EROSION AND SEDIMENT CONTROL

New York Standards and Specifications Page 4.2 August 2005 For Erosion and Sediment Control

There are many techniques used in biotechnical work. Some of the most common are: Vegetated Rock Gabions—This is a combination of vegetation and rock gabions generally used for slope stabilization. Live branch cuttings are layered through the rock gabion structure to anchor in select earthfill. The cuttings protrude beyond the face of the gabion. The gabion standard is covered in the “standard specifications for retaining walls” in Section 5B. See Figure 4.1 for vegetative details. Live Fascines—This technique uses bundles of branches which are staked into shallow trenches, then filled with soil. They are oriented along the contour and are placed in multiple rows to help stabilize a slope. See Standard and Specifications for Live Fascines. Brush Mattress—This method uses hardwood brush layered along a streambank as a mattress and anchored in place with a grid of stakes and wire. The toe below the waterline is anchored by rock. This living blanket acts as a mulch for seedlings and plantings established in the bank. It also prevents erosion of sloped surfaces. See Standards and Specifications for Brush Mattress. Live Staking—These are large stakes or poles sharpened at the bottom end and forced vertically into the soft earth along the waterline, usually about 1 foot apart. Depending on the size of the poles and the composition of the streambank, machinery may be required to force them into the ground or to prepare holes for planting. The poles will grow forming a very thick barrier to flow. See Figure 4.4 and Figure 4.4A. Brush Layering—This technique is generally used to stabilize slope areas above the flow line of streambanks as well as cut and fill slopes. It involves the use of long branches that are placed with cut ends into the slope on bulldozed terraces. The tops protrude outside the finished slope. A layer usually includes three layers of brush separated with a thin (3 in.) layer of soil. On this layer a “lift” of 3-5 feet of soil is placed to form the next terrace and so forth. See Figure 4.5. Live Cribwall—This is a combination of vegetation and structural elements generally used along streams where flowing water is a hazard. Layers of logs are alternated with long branches protruding out between them. The logs are spiked together and anchored into the bank with earthfill behind them to create a wall. The live stems help tie the logs together and screen the wall. See Figure 4.6. Tree Revetment—This method incorporates entire trees (without the root wad) for bank stabilization in areas that are eroded or undercut, but not flashy or in need of heavy maintenance. Trees are overlapped and anchored to the earth for the purpose of absorbing energy and reducing

velocity, capturing sediment, and enhancing conditions for colonization of native species. See Figure 4.7. Branchpacking—This technique alternates live branch cuttings with tamped backfill to repair small, localized slumps and holes in slopes. The alternating layers of branches and soil are placed between long posts driven in to the ground for support. This method is inappropriate for areas larger than 4-feet deep or 6-feet wide. See Figure 4.8. Fiber Roll—A fiber roll is a coconut fiber, straw, or excelsior woven roll encased in netting of jute, nylon, or burlap used to dissipate energy along bodies of water and provide a good medium for the introduction of herbaceous vegetation. This technique works best where water levels are relatively constant. The roll is anchored into the bank and, after suitable backfill is placed behind the roll, herbaceous or woody vegetation can be planted. See Figure 4.9. Properly designed structural measures may be necessary to help protect the toe or face of a slope against scour or erosion from moving water and against mass-moving of soil. These structures are generally capable of resisting much higher lateral earth pressures and higher shear values than vegetation. They can be natural, such as fieldstone, rock and timbers; or, they can be artificial like concrete and steel. Some structural measures can be a combination like gabions, which are wire baskets containing stone. Gabions can be used as retaining walls, grade stabilization structures and slope protection. Many of these types of structures can be planted or vegetated with materials to strengthen the system. See Figure 4.1. Planning Considerations There are many facets that need to be considered when designing a biotechnical system for a site: Method – What is the appropriate method for the particular problem encountered? Materials – What type should be selected? How much is needed to do the job? Where can they be obtained? Schedule – When is the best time to maximize the successful rooting or germination of materials? Equipment – Since this process is somewhat labor intensive, it is necessary to make sure the proper type and amount of tools, such as shovels, pick axe, tile spade, hammers, etc. are available for proper installation of material. Site characteristics – The need for engineering structures will depend on potential hazards, management of site water, soil conditions, and site access. Aesthetics and follow-up

August 2005 Page 4.3 New York Standards and Specifications For Erosion and Sediment Control

maintenance are also important considerations. Protection from livestock is mandatory. Streambanks – Generally applicable where flows are less than 6 feet per second and the stream bottom is not subject to degradation and scour. Protection should be carried to the average high water elevation. Plant Materials

Plant materials for biotechnical slope protection may be obtained in two basic ways. One method is to locate stands of appropriate species and obtain easements to harvest materials from these stands for incorporation into the project. Criteria for selecting native species are: easy rooting; long, straight, flexible whips; and plentiful supply near the site. A second method is to grow and harvest materials from managed production beds that are maintained for commercial distribution. This allows selection of cultivars that have proven performance records and high survival rates. The most popular materials in use today are the shrub willows. Willows have a tremendous ability to sprout roots and stems when in contact with moist soil. Willows are found growing in all parts of the world, so biotechnical

slope protection techniques employ them more than any other group of plants. Two of the tested, proven willow cultivars in the Northeast are:

• ‘Streamco’ purpleosier willow (Salix purpurea) • ‘Bankers’ dwarf willow (Salix cottetii – hybrid)

‘Streamco’ and ‘Bankers’ willow are both shrubs. ‘Streamco’ has an ultimate height of 15-20 feet, while ‘Bankers’ is limited to 6-8 feet. Commercial and state nurseries in the Northeast are producing supplies of both species. In addition to willows, redosier dogwood and poplars are other groups of plants effective for use in biotechnical systems. Species such as elderberry or forsythia can also be used to add biodiversity to a site. All plant materials should be installed on site within 8 hours of cutting, unless provisions for proper storage are made. Materials should be fresh, dormant, and non-desiccated when installed.


Figure 4.1 Vegetated Rock Gabions


STANDARD AND SPECIFICATIONS FOR

LIVE FASCINES

Definition The placement of groups or bundles of twigs, whips, or branches in shallow trenches, on the contour, on either cut or fill slopes.

Purpose To stabilize slopes by slowing water movement down the slope, increasing infiltration, trapping slope sediments, and increasing soil stability with root systems.

Conditions Where Practice Applies On sloping areas such as road cuts, slumped areas, road fills, gullies, and streambanks subject to erosion, seepage, or weathering, which have a low to medium hazard potential should slope failure occur. Slopes must be 1:1 or flatter. Design Criteria Materials—Shall be a native or nursery grown cultivar that is capable of performing the intended function. Fascines—Shall be made by forming the bundles 8-15 feet long, 4 inches minimum in diameter, from stems no more than 1 inch in diameter. Overlap—Fascines should be overlapped at the tapered ends a minimum of 1-foot. Vertical Spacing—The spacing of the contours for the fascines is dependent on the degree of erosion or potential erosion at the site. Factors include slope steepness, soil type, drainage, and existing ground cover. The following is a general guide to selecting contour interval:

Slope Contour Interval

1:1 3’

1.5:1 3’

2:1 4’

2.5:1 4’

3:1 5’

3.5:1 5’

4:1 6’

6:1 8’ See Figure 4.2 for details. Construction Specifications 1. Fascines shall be 4 inches minimum in diameter. 2. Prior to placing the fascines, the slope shall be smoothed

and graded with obstructions removed. Any structural measures for revetment, drainage, or surface water management will be installed first.

3. Working from the bottom of the slope to the top,

excavate the fascine trench. Place fascines in trench and anchor with stakes spaced at 24 inches. Cover fascines with soil leaving about 10% exposed to view. Fascines shall be overlapped 12 inches minimum in the trench.

4. Soil shall be worked into the fascine and compacted by

walking on the fascine being covered. 5. All disturbed areas should be seeded upon completion of

fascine placement. Maintenance Regular inspection and maintenance of fascine installations should be conducted especially during the first year of establishment. Loose stakes should be reset and settled fill areas should be brought back to grade. Prompt corrections to gullies, sloughs or other evident problems shall be made.


Figure 4.2 Live Fascine



BRUSH MATTRESS

Definition A mulch or mattress of brush laid on a slope and fastened down with stakes and wire.

Purpose To protect the soil surface on slopes from erosive forces through the generation of a dense stand of woody vegetation.

Conditions Where Practice Applies Brush mattresses are used primarily on streambanks where the velocity is less than 6 feet per second and excessive runoff from streamflow has created erosive conditions. This practice can resist temporary inundation, but not scour or undercutting. Design Criteria Layer Thickness—The brush shall be a minimum of 3 inches thick (excluding top soil layer). Height—The mattress shall be placed up the bank to the bankfull elevation. The toe of the mattress should be located in a fascine trench.

Slope—The maximum slope shall be 1.5:1. Anchoring—The mattress shall be anchored on the slope by a grid of 3-foot stakes driven on 3-foot centers each way. No. 9 wire is then wound between the stakes, which are driven to secure the mattress. The upstream edge of the mattress should be keyed into the bank 2 feet. Materials—The plant materials should be willow and dogwood brush placed as shown in Figure 4.3. Construction Specifications 1. Prepare slope surface by grading to a uniform, smooth

surface, clear of obstruction. Slopes should be graded before the brush mattress is installed.

2. The fascine toe should be installed first. Then lay

brush beginning at the downstream end of the work. 3. The butt end of the brush will be placed upstream and

plant materials inclined approximately 30 degrees. 4. The upstream edge of the mattress will be keyed into

the slope 2 feet. Stakes will be driven throughout the mattress on 3-foot centers each way beginning along the toe of the mattress.

5. No. 9 wire will be attached to the stakes and tightened

to secure the mattress. 6. Slope areas above the mattress will be shaped and

seeded. Maintenance Scheduled inspections the first year are necessary to make sure the anchoring system is sound. Broken wire or missing stakes shall be replaced immediately. Any missing toe material missing shall be replaced.


Figure 4.3 Brush Mattress



LIVE STAKES

Definition A stake or pole fashioned from live woody material.

Purpose To create a living root mat that stabilizes the soil by reinforcing and binding soil particles together and by contributing to the reduction of excess soil moisture.

Conditions Where Practice Applies Live stakes are an appropriate technique for repair of small earth slips and slumps that are frequently wet and for stabilizing raw streambanks. This technique is for relatively uncomplicated site conditions when construction time is limited and an inexpensive vegetative method for stabilization is derived. It is not intended where structural integrity is required nor to resist large, lateral earth pressures. Design Criteria 1. Live stakes shall be 1 - 2 inches in diameter and 2-6 feet

long, depending on site application. 2. No leaf buds shall have initiated growth beyond 1/4” and

the cambium layer shall be moist, green and healthy. 3. All material shall be maintained in a continuously cool,

covered, and moist state prior to use and be in good condition when installed.

4. Materials harvested on site shall be installed the same day they are prepared. Nursery grown material shall be maintained in a moist condition until installed.

5. Installation Details

a. The lengths of live cuttings/live stakes depends upon the application. If through riprap, the length shall extend through the surface of the stone fill. At least half the length shall be inserted into the soil, below the stone fill.

b. Minimum 2 to 4 inches and two live buds of the live

stake shall be exposed above the stone filling. c. Live stakes shall be cut to a point on the basal end for

insertion in the ground. d. Use a dead blow hammer to drive stakes into the

ground. The hammer head should be filled with shot or sand. A dibble, iron bar, or similar tool shall be used to make a pilot hole to prevent damaging the material during installation.

e. Live cuttings shall be inserted by hand into pilot holes. f. When possible, tamp soil around live stakes. g. Care shall be taken not to damage the live stakes

during installation. Those damaged at the top during installation shall be trimmed back to undamaged condition.

Maintenance Due to the susceptibility of plant materials to the physical constraints of the site, climate conditions, and animal populations, it is necessary to inspect installations frequently. This is especially important during the first year or two of establishment. Plant materials missing or damaged should be replaced as soon as possible. Sloughs or breaks in drainage pattern should be reestablished for the site as quickly as possible to maintain stability.


Figure 4.4 Live Stake


Figure 4.4A Live Stake Construction Specifications

CONSTRUCTION SPECIFICATIONS

1. CARE SHALL BE TAKEN NOT TO DAMAGE THE LIVE CUTTINGS/LIVE STAKES DURING INSTALLATION. THOSE DAMAGED SHALL BE LEFT IN PLACE AND SUPPLEMENTED WITH AN INTACT LIVE CUTTING/LIVE STAKE.

2. THE LENGTHS OF LIVE CUTTINGS/LIVE STAKES DEPENDS UON THE APPLICATION. THE LENGTH

SHALL EXTEND THROUGH THE SURFACE OF THE STONE FILL. AT LEAST HALF THE LENGTH SHALL BE INSERTED INTO THE SOIL, BELOW THE STONE FILL.

3. A PILOT HOLE IS REQUIRED TO ENSURE THAT THE LIVE CUTTING/LIVE STAKE IS NOT DAMAGED

WHEN DRIVEN THROUGH THE STONE FILLING. ACCESS SHALL BE MADE THROUGH THE USE OF A DIBBLE BAR, OR SIMILAR TOOL TO WORK AN OPENING THROUGH THE ROCK LAYER.

4. MINIMUM 2” TO 4” AND TWO LIVE BUDS OF THE LIVE CUTTING/LIVE STAKE SHALL BE EXPOSED

ABOVE THE STONE FILLING. 5. LIVE CUTTINGS SHALL RANGE FROM 1/2” TO 1” IN DIAMETER AND BE FROM 1’ TO 4’ IN LENGTH. 6. LIVE STAKES SHALL RANGE FROM 1” TO 4” IN DIAMETER AND BE FROM 5’ TO 6’ IN LENGTH. 7. SEE CONTRACT DOCUMENTS FOR SPECIES, SIZE, SPACING, LOCATION, AND FINAL

DETERMINATION ON USE OF CUTTINGS OR STAKES. 8. LIVE CUTTINGS/LIVE STAKES SHALL BE CUT TO A POINT ON THE BASAL END FOR INSERTION IN

THE GROUND. 9. USE A DEAD BLOW HAMMER TO DRIVE STAKES INTO THE GROUND. THE HAMMER HEAD

SHOULD BE FILLED WITH SHOT OR SAND. A DIBBLE, IRON BAR, OR SIMILAR TOOL SHALL BE USED TO MAKE A PILOT HOLE TO PREVENT DAMAGING THE MATERIAL DURING INSTALLATION.

10. LIVE CUTTINGS SHALL BE INSERTED BY HAND INTO PILOT HOLES. 11. WHEN POSSIBLE, TAMP SOIL AROUND LIVE CUTTINGS/LIVE STAKES. 12. ANY LIVE CUTTING/LIVE STAKE THAT IS DAMAGED SHALL BE LEFT IN PLACE AND SUPPLE-

MENTED WITH AN INTACT LIVE CUTTING/LIVE STAKE.


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BRUSH LAYER

Definition A brush layer is a horizontal row of live branch cuttings placed in soil with other similar rows, spaced a specific vertical distance apart.

Purpose To stabilize cut and fill slope areas by reinforcing the soil with unrooted branch stems, trap debris on slope, dry excessively wet sites, and redirect adverse slope seepage by acting as horizontal drains.

Conditions Where Practice Applies Generally applicable to stabilize slope areas above the flow line of streambanks as well as cut and fill slopes. Brush layers can be used on slopes up to 2:1 in steepness and 20 feet in height. Design Criteria

The spacing requirements for brush layer rows is dependent on the slope steepness and moisture content. Spacing shall conform with the following table.

Slope Distance Between Layers (feet)

Brush layer cuttings shall be 1/2 to 2 inches in diameter and be from dormant plants. No leaf buds shall have initiated growth beyond 1/4" and the cambium layer shall be moist, green, and healthy. The cuttings shall be long enough to contact the back of the bench with the growing tips protruding out of the slope face. Care shall be taken not to severely damage the live branch cuttings during installation. Damaged cuttings will be replaced prior to backfilling. Starting at the toe of the slope, excavate benches along the contour of the slope. The benches shall range from 2 to 3 feet wide and the surface of the bench shall be angled so the front edge is higher than the back of the bench (See Figure 4.5). The benches shall be spaced according to the previous table, Slope Distance Between Layers (ft). Live branch cuttings shall be placed on the bench in a crisscross or overlapping configuration in layers 3 - 4 inches thick. Backfill shall be placed on top of the live branch cuttings and tamped in 6 inch lifts. Small plate compactors may be used to settle the soil. Areas between the rows of brush layers shall be stabilized by seeding or other appropriate erosion control method. Maintenance Due to the susceptibility of plant materials to the physical constraints of the site, climate conditions, and animal populations, it is necessary to inspect installations frequently. This is especially important during the first year or two of establishment. Plant materials missing or damaged should be replaced as soon as possible. Sloughs or breaks in drainage pattern should be reestablished for the site as quickly as possible to maintain stability.

Slope H : V

Wet Slope

Dry Slope

Max Slope Length

2 to 2.5:1 3’ 3’ 15’

2.5 to 3.5:1 3’ 4’ 15’

3.5 to 4.0:1 4’ 5’ 25’


Figure 4.5 Brush Layer



LIVE CRIBWALL

Definition A hollow box-like structure made with an interlocking arrangement of untreated logs or timber members spiked together and anchored into the slope. The structure is filled with suitable earthfill materials and layers of live branch cuttings which root inside the structure and extend into the slope.

Purpose To protect exposed or eroded streambanks from the erosive forces of flowing water and stabilize the toe of slope to reduce steepness.

Conditions Where Practice Applies Generally applicable where flows are less than 6 feet per second and no degradation of the streambed occurs. Can reduce steepness and provide stability where space is limited and a vertical structure is needed. It is not intended to be used where the integrity of a road or structure is dependant on the cribwall since it is not designed to resist large lateral earth pressures. Design Criteria

1. The vegetated cribwall structure shall be designed to a

height for its intended purpose. 2. Live branch cuttings should be 1/2 to 2 inches in

diameter and long enough to reach from the front of the structure to the undisturbed soil.

3. The structure will be built with a batter of 1 to 12. Large

spikes or rebar are required to secure the logs or timbers together (10 inches minimum).

4. Only untreated logs or timber shall be used in the cribwall.

5. Installation begins with excavating to a stable foundation

2’ - 3’ below the ground elevation at the toe of slope with the back of the excavation (to the slope) slightly deeper than the front.

6. The first course of logs is placed along the front and

back of the excavated foundation approximately 4-5 feet apart and parallel to the slope contour.

7. The next course is placed at right angles on top of the

previous course to overhang the front and back of the previous logs by 3-6 inches.

8. Each course is placed in the same manner and fastened

to the preceding course to the desired grade. 9. Stone fill is placed in the bottom of the structure up to

the ground level and up to the base flow in a stream channel.

10. Once the cribwall structure reaches the existing ground

elevation, live branch cuttings are placed on the stone fill parallel with the slope contour.

11. The cuttings are then covered with select clean fill with

a maximum size of 3 inches and not more than 20 percent passing a 200 sieve size.

12. The live branch cuttings shall be placed at each course

followed by the select fill to the top of the structure with the growing tips slightly protruding from the cribwall face.

13. The plant materials shall be kept in a healthy growing

condition by watering. Also see maintenance below. Maintenance Due to the susceptibility of plant materials to the physical constraints of the site, climate conditions, and animal populations, it is necessary to inspect installations frequently. This is especially important during the first year or two of establishment. Plant materials missing or damaged should be replaced as soon as possible. Sloughs or breaks in drainage pattern should be reestablished for the site as quickly as possible to maintain stability.


Figure 4.6 Live Cribwall



TREE REVETMENT

Definition A tree revetment consists of a tree trunk and branches, without root wad, cabled to an earth anchor, which is buried in the streambank.

Purpose To reduce streambank erosion by absorbing energy and reducing velocity, capturing sediment, and enhancing conditions for planting or colonization of native species.

Conditions Where Practice Applies This practice is appropriate for streambanks that are eroded or undercut. It should not be used near bridges or other structures where there is a potential for downstream damage if a revetment dislodges. Their use should be limited to non-flashy streams where the needs for future maintenance are not important.

Design Criteria

1. Trees shall be sound, recently felled spruce or fir of 6" or greater diameter and at least 20 feet in length.

2. Trees are placed initially at the base flow elevation with

the butt end upstream. Multiple tree revetments shall be overlapped by 25% of their length, working from downstream to upstream.

3. Each tree shall have their branches trimmed off on the

bank side and have two anchors, one near the butt end and the other at 3/4 distance up the trunk.

4. The tree shall be fastened with galvanized cable to the

anchors, which will be commercially manufactured earth anchoring systems. The butt end cable shall also be attached to the stem of the next tree at 3/4 the distance from the base, as it is placed to the outside of the previous tree.

5. Excavate and backfill as necessary to fit the tree

revetment to the site. Maintenance Due to the susceptibility of plant materials to the physical constraints of the site, climate conditions, and animal populations, it is necessary to inspect installations frequently. This is especially important during the first year or two of establishment. Plant materials missing or damaged should be replaced as soon as possible. Sloughs or breaks in drainage pattern should be reestablished for the site as quickly as possible to maintain stability.


Figure 4.7 Tree Revetment



BRANCHPACKING

Definition Branchpacking consists of alternate layers of live branch cuttings and tamped backfill to repair small, localized slumps and holes in slopes.

Purpose The purpose of branchpacking is to provide repair to existing slopes that have small slips or slumps by filling in the failed area with plant materials and soil.

Conditions Where Practice Applies This is an appropriate technique for repairing slip areas that do not exceed 4 feet deep or 6 feet wide. It should not be used as a slope stability measure if structural embankment support is needed.

Design Criteria

1. The live branch cuttings shall be 1/2 - 2 inches in diameter and long enough to touch the undisturbed soil at the back of the area to be repaired. They should extend 4 - 6 inches beyond the finished backfill grade.

2. Wooden posts should be used to secure the plant

material in place. They should be 6 - 8 feet long and 3 - 4 inches in diameter. If lumber is used, it shall be a minimum standard two by four.

3. Wooden posts shall be driven vertically 3 feet deep and

placed in a grid pattern 1 - 2 feet apart. 4. Beginning at the bottom of the slip area, 4 - 6 inch layers

of live branch cuttings are placed in angled layers, 1.5 to 3 feet apart. Compacted moist soil is placed between the layers (see Figure 4.8).



Figure 4.8 Branchpacking



FIBER ROLL

Definition A fiber roll is a coir (coconut fiber), straw, or excelsior woven roll encased in netting of jute, nylon, or burlap.

Purpose To dissipate energy along streambanks, channels, and bodies of water and reduce sheet flow on slopes.

Conditions Where Practice Applies Fiber rolls are used where the water surface levels are relatively constant. Artificially controlled streams for hydropower are not good candidates for this technique. The rolls provide a good medium for the introduction of herbaceous vegetation. Planting in the fiber roll is appropriate where the roll will remain continuously wet.

Design Criteria

1. The roll is placed in a shallow trench dug below baseflow or in a 4 inch trench on the slope contour and anchored by 2” x 2”, 3-foot long posts driven on each side of the roll (see Figure 4.9).

2. The roll is contained by a 9-gauge non-galvanized wire

placed over the roll from post to post. Braided nylon rope (1/8" thick) may be used.

3. The anchor posts shall be spaced laterally 4 feet on

center on both sides of the roll, staggered, and driven down to the top of the roll.

4. Soil is placed behind the roll and planted with suitable

herbaceous or woody vegetation. If the roll will be continuously saturated, wetland plants may be planted into voids created in the upper surface of the roll.

5. Where water levels may fall below the bottom edge of

the roll, a brush layer of willow should be installed so as to lay across the top edge of the roll.



Figure 4.9 Fiber Roll


References 1. Gray, Donald H. and A.T. Leiser. 1982. Biotechnical Slope Protection and Erosion Control. Van Nostrand Reinhold

Company. New York. 2. Dickerson, John A. and D.W. Lake, Jr. 1989. ASAE Paper No. 892 654. Cost Effective Biotechnical Slope Protection

Trials in New York. 3. Dickerson, John A. and M. Van der Grinten. 1991. ICEA paper Biotechnical Slope Protection Systems on Dry Soils in

the Northeast United States. 4. Dickerson, et. al. 1990. ICEA paper. Obtaining Plant Materials for Biotechnical Work.

biotechnical measures for erosion and sediment control (pdf)

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